Throwing it in reverse with rolling ring bearings
Engineers can get rid of clutches, cams, and other mechanical controls by using rolling ring bearings in auto-reversing motion-control systems.
by John Scavitto Product Line Manager, Amacoil Inc. Aston, Pa.
Machines that turn, eject, cut, and spray rely on efficient, automatically reversing linear motion. Rolling ring linear drives are commonly used in these types of applications because they eliminate the need for clutches, cams, gears, and other external controls. However, in many reciprocating, linear motion processes, the speed of the drive must be carefully controlled before and after reversals. Ramping speed down and up lessens the jarring effects on payloads.
Unfortunately, ramping the speed up and down involves complex and costly control systems. Rolling ring linear drives, on the other hand, can be easily adapted to handle the chore by making relatively inexpensive mechanical modifications to the auto-reverse mechanism.
Machined inner race is the key
At first glance, standard ball bearings look just like rolling ring bearings. However, inner races on the rolling ring bearings are machined. Machining standard bearings to make rolling ring bearings is a precise, proprietary procedure. It gives the bearings a contoured, central ridge running around the entire inner race. Ball bearings have perfectly smooth, flat inner races.
When mounted on a shaft, standard ball bearings reduce friction in the hub of rotating assemblies, such as a wheel. Shaft-to-ring contact is across the full surface of its inner race. As the shaft turns, the inner rotating core absorbs friction as the bearings turn on balls in the raceway.
When mounted on a shaft, rolling ring bearings touch the shaft only at the apex of the central ridge on a bearing's inner race. There is clearance between the shaft and bearing on either side of the ridge.
Shaft clearance lets the bearing pivot left and right on the shaft and still maintain point contact. If the inner race were flat, as in standard ball bearings, there would be no clearance, making it impossible to pivot or angle the ring.
When a rolling ring bearing is angled on a rotating shaft, the force generated by the shaft against the central ridge pushes the bearing along the length of the shaft. The rotary input from the motor-driven shaft is thereby converted to linear output.
The housing, or nut, enclosing the rolling ring bearings moves with the rings and carries the payload. The drive's linear direction is determined by the adjustable angle at which the bearings contact the shaft.
In typical rolling ring linear drives three or four rolling ring bearings are inside the drive housing. To reverse the direction of the rolling ring drive, the entire bearing must be flipped to its mirror position on the shaft. The bearing's central ridge provides the pivot on which the bearing assembly is flipped.
On the bottom of the linear drive is a spring-actuated reversal mechanism attached to the rolling ring bearing assembly. When the drive reaches the end of its stroke, the angle of the bearing assembly changes and the drive's direction reverses. End stops can be screws, bolts, bushings, or even small air cylinders. They can be placed on the shaft so that the drive reverses at a specific point.
At no time does the bearing lose contact with the drive shaft. This is how rolling ring linear drives prevent backlash — they eliminate play between shaft and bearing.
Control linear speed without the drive motor
In addition to controlling drive direction, pivoting the bearing to different angles also determines the drive's pitch, that is, the linear distance traveled per shaft revolution. Adjusting the pitch controls linear travel speed relative to each revolution of the linear drive shaft — even if the drive-motor speed remains unchanged. Therefore, a variable-speed system doesn't need clutches, cams, and gears.
For example, increasing the pitch increases the angle of the rolling ring bearing on the shaft. Compression against the bearings' central ridges increases because more of the ridge contacts the shaft. The drive moves faster, and therefore covers a longer linear distance per shaft revolution. Likewise, when pitch is decreased, the angle of the bearing on the shaft decreases. There is less compression against the bearings' central ridges. The drive moves slower and with less linear distance per revolution. It is important to note that these changes in linear speed and linear distance of the nut take place without any adjustments to motor speed or direction of shaft rotation. In most cases, rolling ring drives can use relatively inexpensive, single-speed, unidirectional motors. This type of adjustable pitch control in a linear drive is essential when designing a reciprocating system that has automatic reverse and specific requirements for ramping up and down.
Ramp-down and Ramp-up Configurations
Controlling your reversals
Rolling ring linear drives can be adjusted to meet a variety of ramp down (deceleration) and ramp-up (acceleration) requirements. Adjustable stops installed on the assembly control stroke length. Various hardware fixtures can be attached to the reversal mechanism to control ramping up and down. The most typical application requirements are: ramp down before reversal, ramp up after reversal, and ramp down before reversal then up after reversal.
When linear speed exceeds 9 ips, the simplest and most common device for decelerating rolling ring drives before reversal is a V-cam. This is a simple V-shaped fixture mounted to an adjustable end stop. The V-cam hits the modified reversal mechanism well before the final end-stop reversal point. The reversal mechanism slowly rotates as it rides up the V-cam. The drive's pitch is gradually reduced, decreasing linear speed. By the time the linear drive reaches the end stop and the reversal mechanism is fully flipped, linear speed is almost zero because the ring assembly is almost at its zero-pitch position. Decelerating this way, before reversal, dissipates all of the payload's forward inertia before the drive begins moving in the opposite direction.
A less-expensive device, the K-stop, can ramp down the linear drive prior to reversal. K-stops partially rotate the rolling ring bearing assembly just before the drive reaches the final end stop. With the reversal mechanism partially rotated, the rolling ring assembly moves toward its perpendicular position and the drive's linear speed drops.
K-stops can be configured to rotate the reversal mechanism so that the bearing is perfectly perpendicular to the shaft. This gives the linear drive zero pitch. The drive "dwells" on the rotating shaft with no linear movement until the ring assembly is again angled on the shaft. Air cylinders are often used to activate the reversal mechanism.
At speeds greater than 9 ips, the drive moves too fast for a K-stop to handle. Instead of partially flipping the reversal lever, the lever would completely flip, causing immediate reversal at full speed. That is why, at higher speeds, V-cams are used. V-cams ensure deceleration by beginning a slow, gradual turning of the bearing assembly well before the end stop.
Slow down and ramp up
In applications that need deceleration prior to reversal and then acceleration after reversal, another type of stop, the H-stop, controls the drive-unit pitch. H-stops are attached to the reversal mechanism so that they determine where the reversal mechanism will begin rotating. The H-stop screw catches the leading arm of the H-lever as the linear drive moves, reducing pitch by rotating the ring assembly. The drive thus begins to slow. After the reversal mechanism has been tripped, a second set screw catches the trailing arm of the H-lever, preventing rotation of the ring assembly to its full pitch position.
In this case, reversal is complete but the rings have not pivoted all the way on the shaft and are still held at an acute angle. Therefore, as the drive moves in the opposite direction, it does so at a reduced speed. As the linear drive continues to move, the H-lever gradually pulls away from the stop, and the ring assembly approaches its full pitch position. The linear drive then ramps up to full speed when the reversal mechanism completely clears the second stop.